From the literature (e.g. Ganz et. al., “Measurement of coronary sinus blood flow by continuous thermodilution in man”, Circulation 44:181-195, 1971) it is known that the thermodilution principle can be utilized for in vivo measurements of blood flow in the coronary sinus of human beings. These measurements involve the introduction of a specially designed catheter into the coronary sinus of a patient and injection of a cold indicator fluid from an injection orifice close to the catheter tip. The indicator fluid was flowing along the shaft of the catheter and caused a temperature drop in the blood temperature that was registered by several thermistors coupled in a Wheatstone-bridge arrangement. By knowledge of the indicator temperature and injection rate as well as the measured temperature drop caused by the injected indicator fluid, the coronary sinus flow can be estimated. This type of catheter, with its comparatively large outer diameter, can, however, not be used for measurements of blood flow in individual coronary arteries, and the method suffered also from a rather large variability within the measurements.
The U.S. Pat. No. 6,754,608, which is assigned to the present assignee, reveals that a temperature sensitive sensor mounted at a distal portion of a guide wire could be used for continuously monitoring of the temperature of blood passing the sensor. In use, a guide catheter is introduced to a proximal portion of an artery, and then the sensor guide wire is introduced into the guide catheter and is advanced until the sensor is located downstream of the catheter tip. When cold saline injected into the artery from the open catheter tip passes the temperature sensitive sensor, the sensor will register a temperature drop which is a function of blood flow. This patent is, however, directed to a system for measurements of coronary flow reserve (CFR), wherein a known amount of saline is injected as a bolus, and no suitable measures are taken to adapt the system to measurements of blood flow by continuous thermodilution. A special infusion catheter is, for example, not disclosed. It can further be noted that in the CFR procedure disclosed in U.S. Pat. No. 6,754,608, the temperature is not measured and presented as an absolute value, but instead the temperature drop triggers the start of a time measurement.